The present invention relates to a telephone system operating in time frames each consisting of a plurality of time slots and more particularly to a service generator for generating a plurality of tones.
The signals which are typically required in a telephone system are the ring back tone, dial tone, and trunk busy tone. In addition, other tones are required which are used internally by the telephone system. With the advent of electronic state design and integrated circuits, telephone switching systems are capable of switching pulse code modulated (PCM) digitally encoded signals at an extremely high rate while incorporating a small size.
PCM signals in a telephone switching system enable a multiplicity of conversations to be transmitted over a two wire digitally multiplexed line commonly known as a T1 trunk line. A T1 trunk line multiplexed with other T1 lines forms what is known as a line group. The multiplexed PCM data from the line group is then applied to a time slot interchanger for switching from one time slot or channel of the line group to another time slot or channel of another line group through what is known as space-time-space-division switching techniques.
In an electric switching system for PCM data, the system must be capable of connecting one of several tones to any of the channel time slots. The tones must be digitally synthesized in PCM format to be compatible with the input/output formats. In addition, the digital tones must be compatible with telephone system specifications which may allow only a certain variation from the tone. For example, the American Telephone and Telepraph Company system specifications only allow .+-.0.5% variation from a particular tone.
In the prior art, in order to generate, for example, multifrequency tones, those systems have utilized a memory containing in PCM format a representation of one of the tone pairs for a particular digit. Multifrequency tones are particularly adaptable for tone generation because each multifrequency tone can be represented by 20 PCM samples to accurately represent the particular digit. This is because of waveform symmetry for each multifrequency tone pair. All even quarter periods of the waveform are mirror images of the corresponding odd quarter periods. Also, every half period of the waveform is inverted. In prior art systems, the tones are read out of the multifrequency tone memory cyclically by a 21 state up/down counter. The outputs of the memory are then appropriately complemented and inverted by necessary gating circuitry, depending upon the particular quarter period of the waveform.
In order to generate such tones as dial tone, ring back tone, or trunk busy tone, prior art systems have generally not been able to accurately represent each of the tones with just 20 samples. For example, to accurately represent ring back tone in PCM format requires 200 PCM samples. Similarly, a trunk busy tone requires at least 233 samples. Techniques available in the prior art as described above generally cannot accommodate such requirements.
In accordance with the above background, there is a need for a service generator capable of digitally synthesizing in PCM format those tones, such as ring back, dial, trunk busy, and internal, used in a telephone system compatible with system specifications (e.g., frequency, data format, and other similar characteristics) of currently installed equipment.